In your example (9/20/15 to 11/20/18), would you want the graph to show one value at "3 years and 2 months", or would you want there to be a bar extending from 9/20/15 to 11/20/18?
For the first, you could do a calculation with a subtraction, but using a quarter-year format doesn't really make sense.
For the second, I think you'd want to use a Gantt chart type Build a Gantt Chart
Hope this helps!
It will be all the year quarter combination data, not one value. It will be a cumulative line chart to show the performance. Y- axis is percentage to 100%.
Basically, it will show the pace and percentage.